A filtering device for such a purpose is disclosed in German Offenlegungsschrift No. 2 407 663 and comprises a hollow-cylindrical, rotating filter member. Around the periphery of the filter member, filtering bores are disposed in series above one another.
Back-rinsing bores are formed in a hollow-cylindrical housing which surrounds the rotatable sieve member. As the filter member rotates, the filtering bore moves into and out of alignment with the back-rinsing bores.
The back-rinsing bores are successively opened, as rotation of the filter member continues, by means of a rotatable, cylindrical valve. The valve has transverse bores formed therein and, when the transverse bores form a linear conduit with the respective back-rinsing bore, the valve opens.
There is a drop in pressure of from approximately 250 bars upstream of the filter member to approximately 180 bars within the filter member. Accordingly, the material in the interior of the hollow-cylindrical filter member forces the solid particles, which adhere to the external surface of the filter cylinder, back through the back-rinsing openings and the transverse bores in the cylindrical valve member. The solid particles can then be collected and discharged.
However, there are a number of reasons why such a filtering device has not proved successful in practice. Firstly, when the cylindrical valve member, provided with the transverse bores, is rotated so as to open the back-rinsing conduits, small solid particles can pass into the gap between the cylindrical valve member and the housing. After a very brief period of time, this infiltration of solid material makes it impossible to rotate the valve member. The back-rinsing openings cannot, therefore, be opened and no back-rinsing can be effected. It is necessary to dismantle the entire filtering device before it can be re-used.
Moreover, relatively large solid particles easily block the back-rinsing bores and/or the transverse bores in the cylindrical valve member. In particular, there is a danger that, as the cylindrical valve member continues to rotate, thin-layered solid particles may be drawn into the gap between the housing and valve member. This immediately causes the cylindrical valve member to become blocked.
However, the major disadvantage of such a device is that it has proved extremely difficult to produce a hollow-cylindrical fine filter sleeve with sufficient accuracy to ensure, when high pressure subsists upstream of the sieve member, unfiltered material is prevented from passing into the back-rinsing conduits through the intermediary walls of the receiving chambers formed in the sieve member.
If this disadvantage is not overcome, it is not possible for sufficient pressure to be built up in the interior of the hollow-cylindrical filter member. Such build-up of pressure in the interior of the sieve member is vitally important if back-rinsing and cleaning of the chambers is to be effected. Such chambers are as mentioned hereinbefore, formed in the external sleeve of the filter and accommodate the coarse or large particles.
If, due to leaks, a high pressure cannot be built up in the interior of the hollow-cylindrical filter member, back-rinsing is no longer ensured. Accordingly, the filtering device gradually becomes blocked and no longer functions.